Chern–Gauss–Bonnet theorem
In mathematics, the Chern theorem (or the Chern–Gauss–Bonnet theorem[1][2] after Shiing-Shen Chern, Carl Friedrich Gauss, and Pierre Ossian Bonnet) states that the Euler-Poincaré characteristic (a topological invariant defined as the alternating sum of the Betti numbers of a topological space) of a closed even-dimensional Riemannian manifold is equal to the integral of a certain polynomial (the Euler class) of its curvature form (an analytical invariant).
It is a highly non-trivial generalization of the classic Gauss–Bonnet theorem (for 2-dimensional manifolds / surfaces) to higher even-dimensional Riemannian manifolds. In 1943, C. B. Allendoerfer and André Weil proved a special case for extrinsic manifolds. In a classic paper published in 1944, Shiing-Shen Chern proved the theorem in full generality connecting global topology with local geometry.[3]
Riemann-Roch and Atiyah-Singer are other generalizations of the Gauss-Bonnet theorem.
Statement
One useful form of the Chern theorem is that[4][5]
where denotes the Euler characteristic of M. The Euler class is defined as
where we have the Pfaffian . Here M is a compact orientable 2n-dimensional Riemannian manifold without boundary, and is the associated curvature form of the Levi-Civita connection. In fact the statement holds with the curvature form of any metric connection on the tangent bundle, as well as for other vector bundles over .[6]
Since the dimension is 2n, we have that is an -valued 2-differential form on M (see special orthogonal group). So can be regarded as a skew-symmetric 2n × 2n matrix whose entries are 2-forms, so it is a matrix over the commutative ring . Hence the Pfaffian is a 2n-form. It is also an invariant polynomial.
However, Chern's theorem in general is that for any closed orientable n-dimensional M,[4]
where the above pairing (,) denotes the cap product with the Euler class of the tangent bundle TM.
Applications
The Chern–Gauss–Bonnet theorem can be seen as a special instance in the theory of characteristic classes. The Chern integrand is the Euler class. Since it is a top-dimensional differential form, it is closed. The naturality of the Euler class means that when changing the Riemannian metric, one stays in the same cohomology class. That means that the integral of the Euler class remains constant as the metric is varied and is thus a global invariant of the smooth structure.[5]
The theorem has also found numerous applications in physics, including:[5]
- adiabatic phase or Berry's phase,
- string theory,
- condensed matter physics,
- Topological quantum field theory,
- topological phases of matter (see the 2016 Nobel Prize in physics by Duncan Haldane et al.).
Special cases
Four-dimensional manifolds
In dimension , for a compact oriented manifold, we get
where is the full Riemann curvature tensor, is the Ricci curvature tensor, and is the scalar curvature. This is particularly important in general relativity, where spacetime is viewed as a 4-dimensional manifold.
Gauss–Bonnet theorem
The Gauss–Bonnet theorem is a special case when M is a 2-dimensional manifold. It arises as the special case where the topological index is defined in terms of Betti numbers and the analytical index is defined in terms of the Gauss–Bonnet integrand.
As with the two-dimensional Gauss–Bonnet theorem, there are generalizations when M is a manifold with boundary.
Further generalizations
Atiyah–Singer
A far-reaching generalization of the Gauss–Bonnet theorem is the Atiyah–Singer Index Theorem.[5]
Let be a weakly elliptic differential operator between vector bundles. That means that the principal symbol is an isomorphism. Strong ellipticity would furthermore require the symbol to be positive-definite.
Let be its adjoint operator. Then the analytical index is defined as
- dim(ker(D)) − dim(ker(D*)),
By ellipticity this is always finite. The index theorem says that this is constant as the elliptic operator is varied smoothly. It is equal to a topological index, which can be expressed in terms of characteristic classes like the Euler class.
The GB theorem is derived by considering the Dirac operator
Odd dimensions
The Chen formula is defined for even dimensions because the Euler characteristic vanishes for odd dimension. There is some research being done on 'twisting' the index theorem in K-theory to give non-trivial results for odd dimension.[7][8]
There is also a version of Chen's formula for orbifolds.[9]
History
Shiing-Shen Chern published his proof of the theorem in 1944 while at the Institute for Advanced Study. This was historically the first time that the formula was proven without assuming the manifold to be embedded in a Euclidean space, which is what it means by "intrinsic". The special case for a hypersurface (an n-1-dimensional submanifolds in an n-dimensional Euclidean space) was proved by H. Hopf in which the integrand is the Gauss-Kronecker curvature (the product of all principal curvatures at a point of the hypersurface). This was generalized independently by Allendoerfer in 1939 and Fenchel in 1940 to a Riemannian submanifold of a Euclidean space of any codimension, for which they used the Lipschitz-Killing curvature (the average of the Gauss-Kronecker curvature along each unit normal vector over the unit sphere in the normal space; for an even dimensional submanifold, this is an invariant only depending on the Riemann metric of the submanifold). Their result would be valid for the general case is the Nash embedding theorem can be assumed. However, this theorem was not available then, as John Nash published his famous embedding theorem for Riemannian manifolds in 1956. In 1943 Allendoerfer and Weil published their proof for the general case, in which they first used an approximation theorem of H. Whitney to reduce the case to analytic Riemannian manifolds, then they embedded "small" neighborhoods of the manifold isometrically into a Euclidean space with the help of the Cartan-Janet local embedding theorem, so that they can patch these embedded neighborhoods together and apply the above theorem of Allendoerfer and Fenchel to establish the global result. This is, of course, unsatisfactory The reason that the theorem only involves intrinsic invariants of the manifold, then the validity of the theorem should not rely on its embedding into a Euclidean space. Weil met Chern in Princeton after Chern arrived in August 1943. He told Chern that he believed there should be an intrinsic proof, which Chern was able to obtain within two weeks. The result is Chern's classic paper "A simple intrinsic proof of the Gauss-Bonnet formula for closed Riemannian manifolds" published in the Annals of Mathematics the next year. The earlier work of Allendoerfer, Fenchel, Allendoerfer and Weil were cited by Chern in this paper. The work of Allendoerfer and Weil was also cited by Chern in his second paper related to the same topic.[3]
See also
References
- ^ Gilkey, P.; Park, J. H. (2014-09-16). "A proof of the Chern-Gauss-Bonnet theorem for indefinite signature metrics using analytic continuation". arXiv:1405.7613 [math].
- ^ Buzano, Reto; Nguyen, Huy The (2019-04-01). "The Higher-Dimensional Chern–Gauss–Bonnet Formula for Singular Conformally Flat Manifolds". The Journal of Geometric Analysis. 29 (2): 1043–1074. doi:10.1007/s12220-018-0029-z. ISSN 1559-002X.
- ^ a b Chern, Shiing-shen (October 1945). "On the Curvatura Integra in a Riemannian Manifold". The Annals of Mathematics. 46 (4): 674–684. doi:10.2307/1969203. JSTOR 1969203.
- ^ a b Morita, Shigeyuki (2001-08-28). Geometry of Differential Forms. Translations of Mathematical Monographs. Vol. 201. Providence, Rhode Island: American Mathematical Society. doi:10.1090/mmono/201. ISBN 9780821810453.
- ^ a b c d Schrödinger operators, with applications to quantum mechanics and global geometry. Cycon, H. L. (Hans Ludwig), 1942-, Simon, Barry, 1946-, Beiglböck, E., 1939-. Berlin: Springer-Verlag. 1987. ISBN 978-0387167589. OCLC 13793017.
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: CS1 maint: others (link) - ^ Bell, Denis (September 2006). "The Gauss–Bonnet theorem for vector bundles". Journal of Geometry. 85 (1–2): 15–21. arXiv:math/0702162. doi:10.1007/s00022-006-0037-1.
- ^ "Why does the Gauss-Bonnet theorem apply only to even number of dimensons?". Mathematics Stack Exchange. June 26, 2012. Retrieved 2019-05-08.
- ^ Li, Yin (2011). "The Gauss–Bonnet–Chern Theorem on Riemannian Manifolds". arXiv:1111.4972.
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ignored (help) - ^ "Is there a Chern-Gauss-Bonnet theorem for orbifolds?". MathOverflow. June 26, 2011. Retrieved 2019-05-08.